# If we just undid one rev, use an autosummary
$firstrev = $oldrev->getNext();
- if ( $firstrev->getId() == $undo ) {
+ if ( $firstrev && $firstrev->getId() == $undo ) {
$undoSummary = wfMsgForContent( 'undo-summary', $undo, $undorev->getUserText() );
if ( $this->summary === '' ) {
$this->summary = $undoSummary;